Pendulous accelerometer and other force sensing devices are generally well-know. In particular, pendulous accelerometers are generally well-know as being macro-machined of quartz or another suitable material and having a tuning fork type sensor connected between the proof mass and the frame.
FIG. 1 illustrates a typical generic state of the art pendulous accelerometer or other force sensing device mechanism 1 having a proof mass 2 suspended for out-of-plane motion in response to acceleration. The proof mass 2 is suspended by a flexure 3 between a pair of damping or “cover” plates 4, 5. The generic accelerometer 1 may use gas spring damping, also known as “squeeze film damping,” to smooth out-of-plane motion of the proof mass 2 during exposure to a transitory input, such as a vibratory input.
Damping of the proof mass 2 is provided by the damping plates 4, 5 on either side of the proof mass 2 that are spaced closely enough to form thin gaps 6, 7 between the proof mass and the two damping plates 4, 5. The gaps 6, 7 are filled with a gas that is present in the ambient environment surrounding the accelerometer mechanism 1. Damping of the proof mass 2 is due to compression of the gas and Bernoulli effects within the gaps 6, 7. Damping due to gas compression occurs when the proof mass 2 moves out-of-plane, which causes closing of one gap 6 (or 7) and simultaneous opening of the gaps 7 (or 6). Vibratory input to the accelerometer causes rapid alternate closing and opening of the gaps 6, 7. The damping gaps 6, 7 have no effect when the proof mass 2 is stationary, or moves in response to a steady acceleration input.
In general, the effective damping due to the gas compressibility in each gap is roughly a function of the square of the damper area the cube of the damper gap.
FIG. 2A illustrates an ideal gas spring damper configuration wherein identical gap geometry exists on both surfaces of the proof mass 2, i.e., the heights of the two gaps 6 and 7 are identical, which result in balanced damping forces against both surfaces of the proof mass 2.
However, in a practical accelerometer mechanism 1, inequality in the height of the two gaps 6, 7 can occur, as illustrated in FIG. 2B. The unequal gap spacing affects the behavior of the gas spring damper in an accelerometer where gas compression is the dominating factor.
When the gaps 6, 7 are unequal, more gas compression force is exerted by the narrow gap, and under the application of vibration the proof mass 2 moves to equalize the compression forces in the gaps 6, 7, as illustrated by the arrow in FIG. 2C. The resultant out-of-plane offset of the proof mass 2 is erroneously detected as an acceleration input, which imparts an offset or bias to the output of the accelerometer mechanism 1.
As stated above, the damping force is proportional to the cube of the damping gap dimension h of the gaps 6, 7. This cubic proportionality causes achievement of balanced damping forces to be very difficult in practice due to manufacturing tolerances.
Therefore, devices and methods for overcoming these and other limitations of typical state of the art MEMS accelerometer and other devices are desirable.